-Sex (gender) Determination
Dr. Thomas Hunt Morgan (1866-1945)
-worked with fruit flies (Drosophila
melanogaster)
-pioneer in the use of fruit flies to
study genetics and heredity
Fruit Fly (Drosophila melanogaster)
Phenotypes
Normal Fly
Short Wings
White Eyes
Curly Wings
Yellow Body
Ebony Body
-Early in his work, he viewed
karyotypes of fruit fly chromosomes,
and noticed that male and female
Flies had slightly different
chromosomes.
Fruit flies have 4 pairs of
chromosomes, so the diploid number
is 8.
The Fruit Fly Karyotype
3 of the pairs are homologous, and the
same in male and female.
The 4th pair is different in male and
female.
The 3 pairs that are the same are called
autosomal, and are not involved in
gender determination.
Sex-Linked Traits
-In 1910, Dr. Morgan found that the
trait of white eyes was found mostly
in males (but does happen in females).
-He hypothesized that the recessive
gene for eye color was on the
X chromosome, and that the trait
was sex-linked.
He carried out 2 crosses to test his
hypothesis.
1) Cross a white-eyed male with a
red-eyed female.
Result: 1/2 red eyed females
1/2 red eyed males
So what will the genotypes of
the parents look like?
Xr
Y
XR
XRXr XRY
XR
XRXr XRY
2) Cross one of the F1 red-eyed
females with a red-eyed male
Result: 1/2 red-eyed females
1/4 red-eyed males
1/4 white-eyed males
Start with the genotypes of
the parents.
XR
Y
XR
XRXR XRY
Xr
XRXr X r Y
He predicted that he could produce a
white-eyed female by crossing an F1
red-eye female with a white-eye male.
XRXr
x
XrY
Xr
Y
XR
XRXr XRY
Xr
XrXr
X rY
Many sex-linked abnormalities are
caused by a recessive gene on the X
chromosome.
Examples:
Red-green color blindness
Hemophilia
Nondisjunction
-failure of chromosomes to segregate
during meiosis, resulting in abnormal
chromosome numbers in future
generations.
-Nondisjunction may happen to any
chromosome, autosomal or not.
Nondisjunction in sex chromosomes
can result in many genotypes, such
as:
X
Y (not viable)
XXX (“super females”)
XXY
XXXY (not viable)
The following
diseases are
caused by a
nondisjunction of
the
chromosomes.
Down’s Syndrome:
-caused by a nondisjunction in the
21st pair of chromosomes.
-symptoms include mental retardation,
abnormal facial traits, short arms and
legs, many internal defects.
Downes Syndrome
Turner Syndrome:
-caused by a nondisjunction of sex
chromosomes.
-X genotype (therefore female)
Symptoms: sterile, usually short,
below average intelligence, usually
fail to develop normal female
characteristics.
Turner Syndrome
Klinefelter’s Syndrome
-Individuals have an XXY genotype, so
therefore are male.
-Sterile, usually fail to develop normal
male sex characteristics.
-Very long arms and legs
-Below average intelligence
Klinefelter’s Syndrome
Trisomy 13 (Patau Syndrome)
-a chromosomal condition that is associated with
severe cognitive disability and certain physical
abnormalities.
These abnormalities include: small eyes
-an opening in the roof of the mouth (a cleft palate)
-a cleft lip
-weak muscle tone (hypotonia)
-skeletal abnormalities, including polydactyly
and syndactyly
Affected individuals rarely live past infancy
because of the life-threatening medical problems
associated with this condition.
Patau Syndrome
Polydactyly
Syndactyly
Most cases of trisomy 13 result when each cell in
the body has three copies of chromosome 13
instead of the usual two copies.
A small percentage of cases occur when only some
of the body's cells have an extra copy of chromosome
13, resulting in a mixed population of cells with a
differing number of chromosomes. Such cases are
sometimes called mosaic trisomy 13.
Mosaic trisomy 13 results from a non-disjunction
in a somatic cell during early development.
A young boy (7) suffering
from Patau Syndrome.
He is deaf, and legally blind.
The following
diseases are not
caused by a
nondisjunction in
any chromosomes.
Tay-Sachs Disease
-Affects mostly Jewish people.
-caused by a homozygous recessive
gene.
Symptoms: nervous system does
Not develop normally, causes
inability to move. Death by age
2 or 3.
PKU
-caused by a homozygous recessive
gene.
Symptoms: missing enzyme, can
lead to severe mental retardation.
this condition can be helped with
a special diet. (no phenylalanine)
NO!- Diet sodas, nutra-sweet
Huntington’s Disease
-due to a dominant gene, therefore
a person heterozygous can have the
condition.
Symptoms: deterioration of the brain,
leading to memory loss, and loss
of control of movement.
Cystic Fibrosis
-caused by a recessive gene on the
7th chromosome pair.
Symptoms: lining of lungs does not
produce fluid, causing particles to
be retained in lungs. Chronic cough,
difficulty breathing. Death usually
by 20 years of age.
Sickle-Cell Anemia
-This disease affects mostly African
Americans.
-Homozygous recessive disease.
-Causes irregular shaped red blood
cells,hemoglobin that does not hold
oxygen well.
-Clotting due to cell shape, anemia
due to low affinity of hemoglobin for
oxygen.
In equatorial Africa,
where the disease
originated, it is actually
beneficial to have sickle cell,
because it protects you from having
malaria, which is far more deadly.
Methods of Disease Detection
1) Amniocentesis: Fluid that
surrounds the fetus is withdrawn with
a needle, and analyzed for
chromosome abnormalities
Examples:
Downs Syndrome,Turner Syndrome
Klinefelter Syndrome
DNA needs to be in the form of
chromosomes to be seen clearly.
Sex of fetus can also be determined.
2) Ultrasound: High frequency sound
waves “echo” from the fetus, and give
a picture.
-can be used to see any physical
abnormalities.
EX: limb development, internal organs
3) Fetoscopy:fetus is viewed with
a small camera called an endoscope.
The endoscope is inserted through a
small incision in the mothers
abdomen.
Small samples of tissue or blood may
be taken, some surgical procedures
are now performed.
EX: heart septum repairs, digestive
tract repairs.